Research on the Origin and Early Evolution of Whales (Cetacea)

Introduction

The mammalian order Cetacea is divided
into three suborders: (1) Oligocene to Recent Odontoceti or
'toothed whales'— living today; (2) Oligocene to Recent Mysticeti
or 'baleen whales'— living today; and (3) older and more primitive Eocene
Archaeoceti or 'archaic whales'— which evolved from land mammals and gave
rise to later odontocetes and mysticetes. My research on the origin and early evolution
of whales is focused on archaeocetes. I have been fortunate to work with many
colleagues on this in Egypt, Jordan, Pakistan, and India, (see co-authors in the publication
list below). The stages of early whale evolution that we have documented are
shown here in Figure 1. We have found and collected virtually complete
skeletons of middle-to-late Eocene Basilosauridae (Dorudon and Basilosaurus), exceptionally complete skeletons
of middle Eocene Protocetidae (especially Rodhocetus and Artiocetus), and a partial skull of earliest middle Eocene
Pakicetidae (Pakicetus). Recovery of diagnostic ankle bones in the skeletons of primitive
protocetids during our field work in Pakistan in 2000 confirmed their derivation
from Artiodactyla (the mammalian order including cows, deer, hippos, etc.),
and showed convincingly that whales did not originate from mesonychid condylarths
as Van Valen hypothesized (and we had expected).

Figure 1.
Skeletons of the archaeocetes Dorudon atrox and Rodhocetus
balochistanensis compared to that of Elomeryx armatus,
which is here taken as a model for the extinct group of artiodactyls (Anthracotheriidae,
s.l.) that we now think may have given rise to archaic whales.
Pakicetus has a distinctive skull and lower jaw, but is not demonstrably
different from early protocetids postcranially. Note changes in
body proportions and elongation of feet for foot-powered swimming
in Rodhocetus, then later reduction of the hind limbs and feet
as the tail-powered swimming of modern cetaceans evolved in Dorudon.

Field Work in
Pakistan (1975-1981)

Field work in Pakistan was initiated
in 1975 to investigate sites previously reported as yielding Eocene land mammals.
This led to the suggestion that some fossils there interpreted as mesonychids
might really be archaeocetes (Gingerich, 1977). Our 1977 field work in marine
strata yielded more archaeocetes, which, foolishly, we were not very interested in
at the time (Gingerich et al., 1979). Pelvic bones that we found that year in the Sulaiman Range were
attributed, questionably, to land mammals, because it was impossible to imagine
that whales had such robust hind limbs. In the field we joked that these might
be 'walking whales', but the idea seemed preposterous because there were no
whales known then that were primitive enough to have walked. Our first important contribution was
discovery of the remains of a new archaeocete, Pakicetus inachus, in
the late 1970s. Pakicetus, known only from the skull and lower jaw,
was then the oldest known archaeocete (Gingerich et al., 1983— this distinction
now belongs to Himalayacetus; see Bajpai and Gingerich, 1999).

Field Work in
Egypt (1983-1993)

In the 1980s our field work on archaeocetes
shifted to Egypt, to the classic but long-neglected site of Zeuglodon Valley
or, today, Wadi Hitan. Our camp in the desert in Wadi Hitan is shown in Figure 2,
and investigation of a weathered Basilosaurus is shown in Figure 3. Our
most interesting discovery came in 1989, when we found that both Basilosaurus
isis and Dorudon atrox retained feet and toes (see Figs. 4
and 5).This
discovery then led to renewed investigation of middle Eocene whale strata in
Pakistan, starting in 1991, and focusing on on the Sulaiman Range where we had earlier joked about 'walking whales'.

New whales named from
Wadi Hitan and Fayum Province in Egypt include Ancalecetus simonsi
(Gingerich and Uhen, 1996) and Saghacetus osiris (see Gingerich, 1992).

Field Work in Pakistan again (1991-present)

Our first important find when we
returned to Pakistan in the 1990s was an unusually complete skeleton that we
named Rodhocetus kasranii (Gingerich et al., 1994). This came
from the flank of the Rodho ('bald') part of the Zinda Pir anticlinorium on
the east side of the Sulaiman Range. Rodhocetus is interesting and
important in having a large pelvis connected to the vertebral column, but the
sacral vertebrae in this connection are no longer completely fused, and Rodhocetus
kasranii appears to be an intermediate showing how the sacrum became disarticulated
to make the back flexible as it is in tail-powered swimmers like Dorudon
and later whales. The femur is preserved on one side of the original Rodhocetus
kasranii skeleton, but with this exception, the forelimbs are missing,
the hind limbs are missing, and most of the tail is missing.

Continued
work on the east side of the Sulaiman Range in Pakistan yielded many additional
archaeocetes, including Takracetus simus, Gaviacetus razai,
Dalanistes ahmedi, Qaisracetus arifi, Andrewsiphius sloani,
Babiacetus indicus, Basilosaurus drazindai, and Basiloterus
hussaini (Gingerich et al., 1995, 1997, 2001). However, these specimens
generally lack forelimbs, hind limbs, and tails. Our inability to find limbs
and tails was so frustrating that in 2000 we moved from this area, where fossil-bearing
strata are beautifully exposed, to the west side of the Sulaiman Range in Balochistan
Province. Previously, no fossil whales had been found on the west side of the
Sulaiman Range, and the strata that interest us are not nearly so well exposed
there (Fig. 6).

We have had very good luck finding
well-preserved archaeocetes on the west side of the Sulaiman Range in Pakistan.
The most notable were described and named Artiocetus clavis and Rodhocetus
balochistanensis (Figs. 7-8; see Gingerich et al., 2001). These are
the first early archaeocetes to preserve ankle bones in association with skulls
and skeletons, and the first to show that early whales had distinctively artiodactyl-like
ankles. Thus the earlier idea that whales evolved from mesonychid condylarths
is no longer tenable and we expect that the ancestor was instead something like
an anthracotheriid artiodactyl (e.g., Elomeryx in Fig. 1). From
the point of view of the fossil record, the 'sister-group' relationship of whales
and hippos promoted by molecular phylogeneticists is now plausible, though still
tenuous and unproven.

Combining what we know of the skeletons
of Rodhocetus kasranii and Rodhocetus balochistanensis, it
is possible to make a composite restoration of the latter, which is the reconstruction
shown in Figure 1C.

Field Work in Egypt again (2005-present)

As outlined above, we worked at Wadi Hitan in Egypt from 1983 through 1993, and collected several unusually complete skeletons of 5-meter-long Dorudon atrox. We mapped many skeletons of the 18-meter-long serpentine Basilosaurus isis, but never had the resources to collect a skeleton. Attempts to interpret Basilosaurus have long been frustrated because no one ever collected a complete skeleton anywhere. The classic Basilosaurus cetoides from Alabama restored by Gidley, 1913, and Kellogg, 1936— on exhibit at the Smithsonian Institution in Washington, D.C.— is a composite of two partial skeletons with important parts missing or borrowed from other marine mammals.

Figure 9.
Skeletal restoration of 17-meter or longer fossil whale Basilosaurus cetoides from Alabama published by Gidley (1913).
I prefer this restoration to that of Kellogg (1936) because it looks more lifelike. The true length of
the skeleton is unknown because it is a composite of two partial skeletons. Pelvis and femur here have
been modified to resemble those of Basilosaurus isis described from Egypt (Gingerich et al.,
1991; see above). Figure may be reproduced for non-profit educational use: credit Gidley (1913).

Study of Basilosaurus took a major step forward in 2005 when an important skeleton of B. isis was collected in Wadi Hitan. Later the same year, Wadi Hitan was declared a UNESCO World Heritage Site because of its natural beauty and importance for understanding whale evolution. By prearranged agreement, the new Basilosaurus skeleton is to be prepared, studied, and then returned to Egypt for exhibition. The skeleton is expected to play an integral role in the educational development of Wadi Hitan.

Cetacean Phylogeny

Figure 13.
Phylogeny of Cetacea showing a common ancestry shared with Artiodactyla,
and the hypothesized evolutionary origin of both from older Paleocene age
Condylarthra. Horizontal axis is arbitrary, while the vertical axis is geological
time. Our 2000 discovery of distinctively artiodactyl-like double-pulley
astragalus bones in articulated skeletons of early archaeocetes is the principal
evidence linking whales and artiodactyls as shown here (see Gingerich et
al., 2001). The evolutionary origin of both whales and artiodactyls is closely
tied to the Paleocene-Eocene boundary, and the transition from archaeocetes
to modern whales is related to climatic and ocean circulation changes at
the Eocene-Oligocene boundary. Source: University of Michigan Museum of
Paleontology. Figure may be reproduced for non-profit educational use.

Gingerich Publications on Archaeoceti (chronological order)

Gingerich, P. D. 1977. A small collection of fossil vertebrates from the middle Eocene Kuldana and Kohat formations of Punjab (Pakistan). Contributions from the Museum of Paleontology, University of Michigan, 24: 190-203. PDF (Deep Blue) or Request PDF/reprint 35

Gingerich, P. D. and M. D. Uhen. 1996. Ancalecetus simonsi, a new dorudontine archaeocete (Mammalia, Cetacea) from the early late Eocene of Wadi Hitan, Egypt. Contributions from the Museum of Paleontology, University of Michigan, 29: 359-401. PDF (Deep Blue) or Request PDF/reprint 312

Gingerich, P. D. and M. D. Uhen. 1998. Likelihood estimation of the time of origin of Cetacea and the time of divergence of Cetacea and Artiodactyla. Palaeontologia Electronica, 1(2): 45 pp. [ http://palaeo-electronica.org/1998_2/ging_uhen/issue2.htm]. Online or Request PDF/reprint 331

Gingerich, P. D. 1998. Paleobiological perspectives on Mesonychia, Archaeoceti, and the origin of whales. In J. G. M. Thewissen (ed.), Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea, Plenum Publishing Corporation, New York, pp. 423-449. Order book or PDF or Request PDF/reprint 341

Gingerich, P. D. 2007. Early evolution of whales: a century of research in Egypt. In J. G. Fleagle and Christopher C. Gilbert (eds.), Elwyn Simons: A Search for Origins, Springer, New York, pp. 107-124. PDF or Request PDF/reprint 471

Uhen, M. D. 2004. Form, function, and anatomy of Dorudon atrox (Mammalia, Cetacea): an archaeocete from the middle to late Eocene of Egypt. University of Michigan Papers on Paleontology, 34: 1-222. PDF (Deep Blue: 66-Mb)

Acknowledgments

This research has been funded by the U. S. National Science Foundation, the National Geographic Society, and the University of Michigan. Any opinions, findings, and conclusions or recommendations expressed on this web page are those of the author and do not necessarily reflect the views of the National Science Foundation, the National Geographic Society, or the University of Michigan.